On-line size and composition analysis of particles from ambient aerosols by laser mass spectrometry (LAMPAS)

J. Aerosol Sci., Vol. 26. Suppl 1, pp. S65-$66, 1995

Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00

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On-line Size and Composition Analysis of Particles from Ambient Aerosols by Laser Mass Spectrometry (LAMPAS) K.-P. Hinz*, B. Spengler and R. K a u f m a n n Institute of Laser Medicine, University of DiJsseldorf, P.O.Box 101007, D-40001 Dtisseldorf, Germany.

Keywords: On-line analysis, laser mass spectrometry, ambient aerosol On-line analysis of ambient aerosol particles is currently a major concern in environmental research, industrial hygiene and occupational and environmental medicine. Source identification of combustion products and their relevance for atmospheric reactions, toxicology of inhaled microparticles and clean room control are typical fields of research requiring the exact knowledge of physical and chemical properties of individual particles that are involved. Single particle analysis, instead of bulk analysis, is mandatory since chemical and physicochemical reactions of particles are predominantly a result of individual properties rather than of mean chemical compositions. The major limitations of established techniques for single particle analysis are connected with their off-line mode of operation. Sampling, transportation and preparation of particles considerably increase the risc of artifactual results. Additionally volatile and water-bound constituents can hardly be analyzed this way. To meet the demands of high quality analysis of aerosol particles close to their natural environment the LAMPAS system (LAser Mass analysis of Particles in the Airborne State) was developed/Hinz et al. 1994/. The basic components of the instrument are a differentially pumped interface for direct particle inlet into the ion source region of a time-of-flight mass spectrometer, a particle detection system based on light scattering and a UV laser for elemental and molecular desorption/ionization. Particles sampled directly from ambient air (or for size calibration produced by a nebulizer) pass the inlet system and traverse the main vacuum chamber with their terminal (aerodynamic) velocity /Dahneke 1973/. While crossing a focussed He-Ne laser beam in the center of the ion source region a photomultiplier detects the light scattered by the particle. The discriminated and processed photomultiplier signal is used to actively trigger a UV laser which evaporates and ionizes the detected particle. Ions are accelerated in an electrical field and mass analyzed in a timeof-flight mass spectrometer. The instrument can be operated in positive or negative ion mode. A typical detection and processing rate of 5 particles per minute has been realized. Particle size determination is performed by evaluation of the aerodynamic behaviour of particles. From the known distance between the detection and the ionization laser focus and the delay time between detection and ionization the particle velocity can be calculated. The velocity corresponds to the aerodynamic particle diameter/Cheng et al. 1993/. The light scattering system has successfully been tested and calibrated for particles with diameters between 0.22 I.tm and 5.7 gm using polystyrene latex spheres. After size calibration chemical analysis of particles of a selected size range is possible by choosing an appropriate delay time. The instrument is prepared for fast particle detection and sizing, spectra acquisition, processing and classification and shall be used in stationary field measurements. $65

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Several hundred mass spectra of indoor particles from different aerodynamicaI size classes were registered. They show the individual properties of single particles. Two typical spectra (positive and negative ion mode) from two different size ranges are shown in the figures.

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The positive ion spectrum (Figure 1) is characterized by signals of alkaline salts. Ammonium, carbon and other metal ions are detected as well. The negative ion spectrum (Figure 2) shows characteristic amounts of chlorides, sulfur and nitrogen oxides as well as carbon clusters, indicating a soot particle nucleus. The observation of atmospherically interesting compounds such as hydroxymethane sulfonate from reaction of formaldehyde with sulfur dioxide is notable. For identification and characterization of the main particle classes of an aerosol multivariate calibration methods such as cluster analysis and principal components analysis have been applied. Integration of such methods into future real-time data processing systems will allow for an automated on-line characterization of aerosols in stationary field measurements.

Acknowledgement This work has been supported by the Ministry of Science and Research, Nordrhein-Westfalen, Germany.

References Cheng, Y.S., Barr, E.B., Marshall, I.A., Mitchell, JP.: Calibration and Performance of an API Aerosizer. J. Aerosol Sci. 24, 501-514 (1993) Dahneke, B.: Aerosol Beam Spectrometry. Nature Physical Science 244, 54-55 (1973) Hinz, K.-P., Kaufmann, R., Spengler, B.: Laser-Induced Mass Analysis of Single Particles in the Airborne State. Anal.Chem. 66, 2071-2076 (1994)